Tryptophan, an aromatic amino acid, has been known as a target for protein oxidation induced by radiation, copper ion, and lignin peroxidase. However, oxidative modifications of tryptophan by other kinds of reactive oxygen species (i.e., alkyl-, peroxy- and/or alkoxy-radicals) has not been studied. It is important to identify the oxidative products induced by various kind of reactive oxygen species because this may lead to the understanding of the roles of specific oxidation damage in the etiology and/or progress of aging and age-related diseases. To determine the mechanism of tryptophan oxidation mediated by alkyl-, peroxy-, and/or alkoxy-radical, a tri-peptide, alanine-tryptophan-alanine (A-W-A), was incubated with azo-bis amidinopropane HCl (AAPH), which decomposes in aqueous solution to generate alkyl- and peroxy-radicals in the presence of oxygen. A-W-A, was progressively modified during oxidation as indicated by disappearance of the original peptide peak. At 5 and 28 hours after incubation, 28% and 95% of A-W-A were modified respectively. The main product formed in the oxidation process was kynurenine, which is determined by comparing its UV absorbance spectrum with kynurenine standard. Twelve percent of A-W-A was converted to kynurenine after 28 hours incubation with AAPH. In addition to kynurenine, some other products were also observed. Further study will be undertaken to identify the unknown products in this oxidation system.